MGM fact sheet: introduction

Mechanics of Granular Materials

Second round of space experiments on STS-89 will help usunderstand better the behavior of soils, powders, and other
solid particlesunder very low confining pressures

Anyone who has ripped open a vacuum packed pouch
of coffee has experienced a fundamental aspect of granular mechanics: a
singular shift in conditions can drastically change the properties of a
bulk material. While the atmosphere presses on the pack, the grains push
against one other, locking each other in place, creating a stiff "brick."
Once pressures are released, the grain assembly becomes very weak and soft,
and moves about freely, almost like a liquid.

During critical, unstable states - like liquefaction of saturated, loose
sand during an earthquake - gravity acts as a "follower load"
that makes the structure collapse. Even under laboratory conditions, this
is too rapid to allow detailed study of intergranular forces and conditions.
Further, gravity-induced stresses complicate the analysis.

To understand how granular materials behave under low stresses, NASA
has sponsored the Mechanics of Granular Materials (MGM) experiment for flights
aboard the U.S. Space Shuttle. In orbit, MGM uses the weightless environment
of orbital flight to test soil under very low pressures. The results will
further understanding of the behavior of granular materials and help in
conceptual and analytical modeling. This will be applied to improving foundations
for buildings, managing undeveloped land, and handling of powdered and granular
materials in chemical, agricultural, and other industries.

Geologic processes are closely related. Wind and river
transport processes are controlled by the properties of cohesionless granular
assemblies. Liquefaction phenomena observed in unconsolidated and cohesionless
soil deposits during earthquakes are governed by constitutive, dilatancy,
and stability properties. When intergranular stresses or pressures become
very low, as during earthquake-induced liquefaction, the soil-water composite
momentarily acts like a viscous liquid, allowing buildings to sink and tilt,
bridge piers to move, and buried structures to float.

Above left: An automobile lies
crushed under the third story of this apartment building in the Marina District
after the Oct.
17, 1989, Loma Prieta earthquake. The ground levels are no longer visible
because of structural failure and sinking due to liquefaction. Links to
768x512, 224K JPG. Credit: J.K. Nakata, U.S. Geological
Survey.

Above right: Ground shaking
triggered liquefaction in a subsurface layer of sand, producing differential
lateral and vertical movement in a overlying carapace of unliquified sand
and silt, which moved from right to left towards the Pajaro River. This
mode of ground failure, termed "lateral spreading," is a principal
causet of liquefaction-related earthquake damage caused by the Oct. 17, 1989,
Loma Prieta earthquake. Links to 768x512, 224K
JPG. Credit: S.D. Ellen, U.S. Geological Survey.

The first flight of MGM, on STS-79
(September 1996) was highly successful. One of the main findings is that
the lower the confining pressure on the dense specimens, the higher the
friction angle becomes (i.e., the specimens become stiffer). The second
flight, scheduled for STS-89 in January 1998, will comprise twice as many
experiment runs and expanded test conditions.

MGM traces its origins to studies to help design the wire mesh wheels
for the Lunar Rover Vehicles driven by astronauts on the last three Apollo
missions in 1971-72. Results from MGM may be applied in the future to advanced
rovers for the exploration of Mars in addition to terrestrial needs.

Key personnel

Principal Investigator: Dr.
Stein Sture, University of Colorado at Boulder.